Phased array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front or cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The differences in phase or timing among the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Calibration of phased arrays may be performed during the manufacturing process using near-field or far-field sources. Calibration of phased arrays after fielding may be performed using near-field or far field sources, or by internally distributed reference calibration signals. In general the near-field and far-field scanning process for initial calibration can be very time consuming, especially for arrays with large numbers of elements. Often, typical calibration and maintenance procedures require the antenna to be taken out of service or offline in order to undergo phase and amplitude calibration. Hence, recalibration after operational deployment is only performed when necessary to compensate for defective elements, compensate for changes in element performance over time, temperature or other influencing factors, maintain desired radiation pattern characteristics, implement antenna changes, and maintain overall peak performance, for example.
Prior art phased array calibration techniques using a calibrated internally generated and distributed test signal add cost, weight and complexity to the system. Other calibration techniques have used external probes which require external hardware, add cost, weight and complexity to the system and can be subject to multipath reflections and external interference. They may also be unsuitable for tactical equipment.
Still other prior art attempts to overcome the above mentioned problems have involved the use of mutual coupling measurements, whereby the inherent mutual coupling among radiating elements is utilized to perform an on-board, automatic calibration procedure on the array without taking the antenna out of service. Two previous publications disclosing such prior art mutual coupling calibration techniques are entitled “Phased Array Antenna Calibration and Pattern Prediction Using Mutual Coupling Measurements” (Herbert M. Aumann et al., IEEE Transactions on Antennas and Propagation, Vol. 37, No. 7, pp. 844-850, July 1989), and “Mutual-Coupling-Based Calibration of Phased Array Antennas” (Charles Shipley et al., IEEE 0-7803-6345-0/00, pp. 529-532, 2000). With reference to the schematic illustration of FIG. 1 showing elements in a phased array antenna system 100, these prior art calibration measurements utilizing mutual coupling require a transmit element 10 within the array 100 along with symmetrically opposed receiving elements 20, 30 having equal amplitude and phase mutual coupling to element 10. The amplitude and phase of the transmit signal from element 10 is received sequentially by elements 20, 30 in their zero amplitude and phase bit settings. Based on the relative measurements, transfer functions are then calculated relating the gain and phase of elements 20 and 30. The calibration coefficients for the phased array antenna system are then derived based on the determined transfer functions.
However, the prior art includes a number of drawbacks and limitations associated with the present mutual coupling calibration implementations. Calibration measurements require signals within the linear dynamic range of the receive elements. The prior art techniques indicate use of nearest or near neighboring symmetrically opposed receive elements. However, full power transmit signals may not be within the linear dynamic range of near neighboring receive elements, resulting in distorted or ineffective array calibration over a wide band of signal energy levels. In addition, the prior art solutions include accuracy limitations in that neighboring elements may have very closely matching gain and phase values, while the array calibration measurements may be required to resolve intensity differences of fractions of a decibel (dB) or less and phase differences of only a few degrees. A system and method which overcomes the aforementioned difficulties is highly desired.